The resonator fiber optic gyroscope (RFOG) has tremendous potential of meeting the needs of many navigation and attitude control markets and creating new markets. The reason for this great potential is the RFOG has promise of being the lowest cost and smallest sized gyro for a high performance rotation sensing device. One problem of the RFOG achieving the required performance levels is double backscatter or double back-reflections within the resonant ring, which introduces an error in rotation sensing.
RFOGs work by measuring the resonance frequency shift of a ring resonator between two counterpropagating waves that are directed to propagate within the ring resonator. The resonant frequency shift is proportional to the rate of inertial rotation about an axis normal to the effective plane in which the resonator is formed. The resonator for an RFOG contains optical fiber for a substantial portion of its round-trip pathlength. For accurate measurement of the resonance frequencies of an RFOG, and hence the rotation rate, light waves coupled into the resonator in clockwise (CW) and counter clockwise (CCW) directions are required to propagate substantially independently in the CW and CCW (and substantially in only their intended direction, hence termed “signal waves”) inside the ring resonator.
In practice, however, optical surfaces, or interfaces inside the resonator may direct a fraction of each of the light waves into the opposite propagation direction. One class of cases of these unwanted imperfections is when a fraction of light from one wave is reflected into the opposite direction through an odd number of reflections. This produces a parasitic wave of light that was, for instance, derived from CW propagating light in the resonator that eventually (undesirably) propagates in the CCW direction. This parasitic was produces optical interference effects between the intended, CCW “signal” wave (that has propagated solely in the CCW direction) and the parasitic wave in the CCW direction. The same phenomena may also occur by light reflected an odd number of times that originated in the CCW direction, producing a parasitic wave in the CW direction that interferes with the “signal” wave in the CW direction. The impact of resulting interference in either the CW or CCW directions on rotation measurement (caused by odd number of reflections in the resonator) may be greatly attenuated if the CW and CCW light waves are separated in wavelength significantly and/or modulated and demodulated at different frequencies. However, a second class of these imperfections also exists, that is, if lightwaves are reflected by an even number of reflections. The technique used to mitigate the first class of unwanted imperfections will not work because the reflected parasitic light wave will have the same wavelength and/or modulation frequency as the primary “signal’ light wave, since it is derived from the primary “signal” light wave and cannot be rejected through the demodulation process. A light wave that travels along a path that involves being reflected twice (involving two reflections, which produces the highest intensity of reflected-waves among those through even number of reflections) forms a parasitic interferometer with the signal light wave and creates distortions of the resonance line shape. These distortions lead to bias errors in the measured rotation rate. Finding methods to reduce such errors stemming from such spurious double reflection paths is important for improving the RFOG performance.